Lawmakers want to reconstruct the digital trail left by Omar Mateen in the months before his attack at an Orlando nightclub early Sunday that left 49 people dead.
The Senate Homeland Security Committee has asked for Facebook's help in tracing Mateen's use of the site — including whether he searched the social network during the June 12 shooting.
The letter, signed by Republican Sen. Ron Johnson, chairman of the Committee on Homeland Security and Governmental Affairs, says that Mateen may have used as many as five Facebook accounts. The committee wants Facebook to provide information on Mateen's Facebook accounts and activities — including messages, photos, activity logs and other data.
Facebook confirmed on Thursday that it had received the letter. The company implemented its "Safety Check" feature for the first time in the U.S. in the wake of the shooting at the Pulse nightclub.
"It is my understanding that Omar Mateen used Facebook before and during the attack to search for and post terrorism-related content," Johnson wrote in the letter dated June 15 and addressed to Facebook CEO Mark Zuckerberg.
"On June 12, 2016, Mateen apparently searched for 'Pulse Orlando' and 'Shooting.'"
Johnson also requests Facebook's help in determining whether Mateen posted messages expressing pro-ISIS sentiments on any of his accounts. The letter portrays Mateen as using Facebook as a research tool in the months before the shooting, and says he showed an interest in the terrorist couple who killed 14 people in San Bernardino, California last December.
A senior law enforcement official told NBC News that FBI analysts believe Mateen posted the statements praising ISIS on a Facebook account controlled by him, including one post they believe was made around the time he made a 911 call from within the club.
"You kill innocent women and children by doing airstrikes ... now taste the Islamic state vengeance," he wrote in that post.
"In addition, my staff learned that in May 2016, Mateen used Facebook to search for information on the San Bernardino terrorists and on June 4, 2016, Mateen apparently searched 'Baghdadi Speech,'" Johnson wrote. "My staff has also learned that Mateen apparently used Facebook to conduct frequent law enforcement and FBI searches, including searching for specific law enforcement offices."
Johnson asks for Facebook to provide the requested information by June 29.